The battery change interval for an EPIRB or SART is typically 5 years. Irrespective of whether the beacon flashes its light when you press the test button, you need to check the service date and get the battery changed on time. This will ensure the unit will operate in an emergency, and will have enough remaining battery life to keep working until you are rescued.
EPIRBs on SOLAS vessels are subject to mandatory shore-based maintenance at 5 year intervals under SOLAS IV/15.9. IMO guidelines here
Test equipment supplied by Sartech also needs to be calibrated every 2 years to ensure the validity of test results.